An automatic transmission of a vehicle may include a torque converter, a planetary gear train connected to the torque converter that functions as a multi-speed mechanism, and a hydraulic control system for operating any one operation element among operation elements of the planetary gear train according to a running state of the vehicle.
The automatic transmission may include the planetary gear train including a compound planetary gear set formed by combining at least two simple planetary gear sets to achieve required multiple speed stages and a plurality of friction elements, and the hydraulic control system may selectively operate the friction elements of the planetary gear train according to a driving condition.
Various solenoid valves are used in the hydraulic control system and suppress electric consumption when a vehicle runs. In order to prevent undesired friction elements from being engaged when the automatic transmission is out of order (fails), solenoid valves of normally closed type are used.
A normally closed type solenoid valve is a solenoid valve configured not to output hydraulic pressure when current is not applied to the solenoid valve.
In further detail, current is applied to a necessary solenoid valve and hydraulic pressure is supplied to friction elements necessary to achieve forward speed stages or reverse speed stages when a shift range is changed to a forward range or a reverse range by means of manipulation of a shift lever.
However, current is not applied to the solenoid valve and the reverse speed stage is not achieved when the hydraulic control system is out of order even though shifting to the reverse speed stage is demanded.
Disconnections, short circuits, and failures of shift range sensor (in this case, a shift range cannot be detected and it cannot be determined which solenoid valve current is applied to) are examples of failures where the current is not applied to corresponding solenoid valves.
For preventing operation of undesired friction elements, the hydraulic control system is designed to enter a solenoid all off failure mode when the automatic transmission fails. In the solenoid all off failure mode, current is not applied to any one of the solenoid valves of the hydraulic control system.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.